wibo
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Posts: 26
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Post by wibo on Oct 19, 2015 2:58:25 GMT -8
Hi All, Since there is no chimney in the shed my heater is for, one needs to be installed. I'm not keen on going through the roof. I'm happy it's waterproof as it is. So I plan on going through the wall and than up. (I still might change my mind on that.. Current heater is roughly 4"system size, but I would like to be able to upgrade / experiment with potentially bigger stoves. The idea is to install a 6" chimney going out and up and connect with the correct tubing to that chimney. Theoretically I would expect draft to be improved by using a bigger chimney than system size. Here's an interesting page on draft calculation for those interested. www.engineeringtoolbox.com/natural-draught-ventilation-d_122.htmlSo, question is: am I right in assuming installing a bigger than needed (diameter) chimney will improve draft or am I missing something?? Regards, Wicherd
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Post by satamax on Oct 19, 2015 3:54:26 GMT -8
Well, it depends, for a it will take more time for a given volume of gases, to travel a bigger pipe than a smaller pipe. So if your pipe is realy big compared to the original, friction and low velocity will unduce turbulences, and might draw worse. Well, that would be trhe case with a 4 incher feeding a 16 inch chimney for example.
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wibo
New Member
Posts: 26
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Post by wibo on Oct 19, 2015 6:35:43 GMT -8
That's an interesting perspective to look at things. If I understand you correct it is depending on whether the gasses are being pushed out, or pulled out; Entering the bigger pipe, gasses will slow down. So now the question becomes if it's the big pipe that generates the draft and starts pulling on the gasses in the small pipe (increased draft / pull?), or the short small pipe that tries to push the air out and now the big pipe makes it harder to push the air out.
I thought I was slowly starting to get it... Not. I have some 150mm pipe and some 100mm pipe. Maybe I can do some sort of an experiment. Setting it up is simple enough. I just have no idea how to actually measure the resulting draft..
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Post by satamax on Oct 19, 2015 10:26:05 GMT -8
Nah nah nah Wibo. It doesn't work like this. Cold air is heavier than hot air, or gases for that instance. So hot air gets pushed up by the cold air. A bit like if you pour water in diesel. the water drops down below the diesel. Here there's no push or pull realy involved. Well there is, but that the main stack effect. en.wikipedia.org/wiki/Stack_effectHere what you have involved is volumes. Let say 10 liters occupy 56.58cm of lengh of a 150mm pipe While the same 10 liters occupy only 31.83cm of lengh in a 200mm pipe. Which means the volume of gas expelled by a 15cm rocket won't keep the same velocity in the 200mm pipe, it will get slower. And also, the circumference is bigger, leading to more friction on already slowing gases. Creating turbulence, and also an effect not negligible, the cooling. Since you have more time to exchange the heat, due to the slowing down of the gas column; with the surounding pipe. And that surounding pipe has more exchange surface. You cool your gases more. This could lead to stalling in an already poorly working system. On the other hand, it can be very intresting to use this on a verry drafty system, to gather more calories before it exits the building. There's a good trick, small pipe out of the rocket, same as system size. Then bigger pipe, to increase the time and exchange surface to extract the calories. Then back to a small pipe to exit the house, to gain a bit more velocity. Mind you, each time you increase or decreease the size of the pipe, you have energy losses. Mainly due to friction and turbulence. But friction re heats the gases, so not all is lost
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wibo
New Member
Posts: 26
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Post by wibo on Oct 20, 2015 3:47:45 GMT -8
Looks like I'm stuck on system size than... Not sure how tight the tolerances are: Got a slightly overdimensioned core, 9x9 cm. 81 cm 2. A 100mm pipe is 78 cm 2 That would be to small... Next diameter is 110mm, 95 cm 2 (thin walled metal, welded seam) not to great on quality, but cheap though not insulated. and after than 120mm, 113 cm 2 (in stainless, single and double wall pipe and in a 2mm steel version) I would not mind paying a little extra for insulated pipe (100 or 120mm) Question is the diameter... what happens if I go for 120 mm directly after the stove (barrel that is..) maybe into a next bell to catch some extra heat, and than out via the actually "undersized" 100mm tube Insulated, up and out..
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Post by peterberg on Oct 20, 2015 5:33:10 GMT -8
Beware of a false comparison between square and round pipes in a system. Aerodynamics properties are much lower in squares, so as a rule of thumb it's best to say a 100 mm square has the same properties as a circle of 100 mm diameter. In fact, the corners are forming drag "holes".
So, your 90 mm square is in fact smaller as compared to a 100 mm circle. But don't worry, recently I have tried 100 mm square risers in tiny batch box systems coupled to a 120 mm bare chimney pipe and those ran beautifully. A 150 mm pipe as chimney diameter for an undersized 100 mm system is probably crossly oversized in my opinion.
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wibo
New Member
Posts: 26
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Post by wibo on Oct 20, 2015 13:09:05 GMT -8
Beware of a false comparison between square and round pipes in a system. Aerodynamics properties are much lower in squares, so as a rule of thumb it's best to say a 100 mm square has the same properties as a circle of 100 mm diameter. In fact, the corners are forming drag "holes". I did read that before somewhere, just didn't think that would apply to the burntunnel to... guess i'll start with some simple bare 120mm piping. cheap enough to do some experimenting. Like the idea of going straight out the roof better than having 2 90 degree turns. more work to be done.. Thanks for the advise gents. And Peter, your welcome to come and measure things when all's done.. we live pretty close
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